Polylactic acid molding and process for producing the same

ABSTRACT

A polylactic acid formed article and a production process therefor. A resin composition for the formed article comprises polylactic acid having an optical purity of not lower than 90% and a residual lactide amount of 0.1 to 0.6% by mass, and 1 to 25% by mass of a crystal nucleus agent. The formed article is characterized in that a difference (|ΔHm|−|ΔHc|) between the absolute value of a crystal fusion heat amount ΔHm as measured at a heat-up rate of 20° C./min by means of a differential scanning calorimeter and the absolute value of a heat-up crystallization heat amount ΔHc generated by heat-up crystallization is not lower than  25  J/g, and has a crystallinity of not lower than 35% as determined by X-ray measurement and a crystallization speed of not lower than 0.05 min −1  at 130° C.

TECHNICAL FIELD

The present invention relates to a polylactic acid formed article and aprocess for producing the same.

BACKGROUND ART

With a recent increasing social demand for environmental conservation,biodegradable polymers which are decomposed by microorganism haveattracted attention. Specific examples of the biodegradable polymersinclude fusion-moldable polyesters including aliphatic polyesters suchas polybutylene succinate, polycaprolactone and polylactic acid, andaliphatic/aromatic copolymer polyesters such as terephthalicacid/1,4-butanediol/adipic acid copolymers. Among these aliphaticpolyesters, the polylactic acid, which is widely distributed in natureand is harmless to animals, plants and humans, is highly heat-resistantwith a melting point of 140 to 175° C. The polylactic acid is promisingas a less expensive thermoplastic biodegradable resin.

Where the polylactic acid is molded or formed into a sheet or acontainer, however, crystals of the polylactic acid are generallycompletely fused due to thermal history experienced during the moldingor the forming, so that the resulting molded or formed article is poorin heat resistance.

Many attempts to impart the polylactic acid with heat resistance havebeen reported. For example, JP-A-8-193165 proposes a method forproducing a molded article by injection-molding, blow-molding orcompression-molding a polylactic acid polymer to which talc, silica orcalcium lactate is added as a crystal nucleus agent. However, thismethod is problematic in that the crystallization of the polymer isinsufficient without a heat treatment and the productivity is low with alower crystallization speed of the polymer. Further, JP-A-4-220456proposes a method in which polyglycolic acid and its derivative areadded as a crystal nucleus agent to poly-L-lactide to increase thecrystallization speed for reduction of an injection molding cycle timeand to improve the mechanical characteristics of the resulting moldedarticle.

However, JP-A-8-193165 states that an attempt was made to produce amolded article by the injection molding by the method stated inJP-A-4-220456 but failed at a mold temperature of not lower than Tg asdisclosed in JP-A-4-220456.

JP-A-11-106628 discloses a method in which wax is employed as a crystalnucleus agent and a crystallization promoter, and a molded article isheat-treated at a crystallization temperature or kept in a mold set atthe crystallization temperature for a predetermined period. However, thewax employed as the crystal nucleus agent is generally less compatiblewith the polylactic acid thereby to be bled out. Therefore, only a smallamount of the wax is added, which is insufficient for formation ofcrystal nuclei.

JP-A-9-25345 discloses a method for imparting heat resistance and shockresistance without the use of a crystal nucleus agent, wherein anunstretched sheet is stretched 1.5 to 5 times for improvement of thecrystal orientation and crystallinity of the sheet. However, the sheetproduced by this method is a stretched sheet, which is further stretchedwhen subjected to a forming process to provide a formed article.Unfortunately, the sheet once stretched is inferior in drawability and,therefore, is not suitable for deep drawing. The applications of thesheet are inevitably limited.

DISCLOSURE OF THE INVENTION

To solve the aforesaid problems, it is an object of the presentinvention to provide a polylactic acid formed article which is excellentin heat resistance and shock resistance and can be produced at a higherproductivity by a forming process.

According to a first aspect of the present invention to achieve thisobject, there is provide a polylactic acid article formed from a sheetof a resin composition comprising polylactic acid as a major resincomponent and a crystal nucleus agent, the polylactic acid having anoptical purity of not lower than 90% and a residual lactide amount of0.1 to 0.6% by mass, the crystal nucleus agent being present in aproportion of 1 to 25% by mass in the resin composition, the formedarticle being characterized in that a difference (|ΔHm|−|ΔHc|) betweenthe absolute value of a crystal fusion heat amount ΔHm as measured at aheat-up rate of 20° C./min by means of a differential scanningcalorimeter and the absolute value of a heat-up crystallization heatamount ΔHc generated by heat-up crystallization is not lower than 25J/g, the formed article having a crystallinity of not lower than 35% asdetermined by X-ray measurement and a crystallization speed of not lowerthan 0.05 min⁻¹ at 130° C.

Although the polylactic acid is known as a material having a very lowcrystallization speed, the crystallinity (crystallization speed) of thepolylactic acid per se is increased by limiting the optical purity andresidual lactide amount of the polylactic acid as described above.Further, the crystallinity of the polylactic acid after the forming isincreased by the addition of a proper amount of the crystal nucleusagent. Thus, the formed article has an excellent heat resistance.

According to the present invention, the crystal nucleus agent ispreferably talc having an average particle diameter of 0.1 to 10 μm.

According to the present invention, the resin composition preferablyfurther comprises a dispersant for the crystal nucleus agent, and thedispersant preferably comprises an aliphatic amide.

The aliphatic amide preferably comprises at least one of erucamide,stearamide, oleamide, ethylene-bis-stearamide, ethylene-bis-oleamide andethylene-bis-laurylamide.

The inventive polylactic acid formed article is preferably formed by oneof vacuum forming, air pressure forming, vacuum air pressure forming andpress forming.

A first production process for the polylactic acid article formed fromthe sheet of the resin composition comprising the polylactic acid as themajor resin component and the crystal nucleus agent according to thefirst inventive aspect comprises the steps of: extruding the resincomposition into a sheet, the polylactic acid in the resin compositionhaving an optical purity of not lower than 90% and a residual lactideamount of 0.1 to 0.6% by mass, the crystal nucleus agent being presentin a proportion of 1 to 25% by mass in the resin composition;heat-treating the sheet at a temperature of 110 to 150° C. for 1 to 30seconds; and forming the sheet into the article after the heattreatment.

A second production process for the polylactic acid article formed fromthe sheet of the resin composition comprising the polylactic acid as themajor resin component and the crystal nucleus agent according to thefirst inventive aspect comprises the steps of: extruding the resincomposition into a sheet, the polylactic acid in the resin compositionhaving an optical purity of not lower than 90% and a residual lactideamount of 0.1 to 0.6% by mass, the crystal nucleus agent being presentin a proportion of 1 to 25% by mass in the resin composition; andforming the sheet into the article while heat-treating the sheet at atemperature of 110 to 150° C. for 1 to 30 seconds.

According to the present invention, one of vacuum forming, air pressureforming, vacuum air pressure forming and press forming is preferablyemployed for the forming.

The polylactic acid formed article according to the first inventiveaspect is formed from the sheet comprising the crystal nucleus agentblended in the specified amount in the specified polylactic acid, sothat the crystallization speed of the resin composition is promoted. Theformed article has an excellent heat resistance such that the difference(|ΔHm|−|ΔHc|) between the absolute value of the crystal fusion heatamount ΔHm measured at a heat-up rate of 20° C./min by means of thedifferential scanning calorimeter and the absolute value of the heat-upcrystallization heat amount ΔHc generated by heat-up crystallization isnot lower than 25 J/g, the crystallinity determined by X-ray measurementis not lower than 35%, and the crystallization speed is not lower than0.05 min⁻¹ at 130° C.

In the production processes for the polylactic acid formed articleaccording to the first inventive aspect, the resin compositioncomprising the specified polylactic acid and the specified amount of thecrystal nucleus agent is formed into the sheet, and the sheet isheat-treated under the specified conditions prior to or simultaneouslywith the sheet forming. Thus, the polylactic acid formed articleaccording to the present invention can easily be produced.

The polylactic acid formed article is advantageously applicable tocontainers required to have a heat resistance, for example, tablewaresuch lunch trays, bowls, dishes and cups. Since the polylactic acidformed article is free from deformation during storage or transportationthereof in summer, the polylactic acid formed article has a variety ofapplications for lid materials, building materials, boards, stationery,cases, carrier tapes, cards such as prepaid cards and IC cards, FRPs andvarious containers. Since the polylactic acid formed article is mainlycomposed of the biodegradable polylactic acid, the polylactic acidformed article is not accumulated in a natural environment even if beingdiscarded after use. This makes it possible to alleviate anenvironmental burden to the natural environment and wild animals.

According to a second aspect of the present invention, there is provideda polylactic acid article formed from a sheet of a resin compositioncomprising polylactic acid as a major resin component, the resincomposition comprising a crystalline polylactic acid resin (A) having anoptical purity of not lower than 95%, an aromatic/aliphatic copolymerpolyester or an aliphatic polyester (B) having a glass transitiontemperature of not higher than 0° C., and talc (C) having an averageparticle diameter of 1 to 8 μm with an (A)/(B) blend ratio of(A)/(B)=97/3 to 80/20% by mass and with a (C) blend ratio of 1 to 30% bymass based on the total amount of the composition, the formed articlehaving a crystallization index such that a difference between theabsolute value of a crystal fusion heat amount ΔHm as measured at aheat-up rate of 20° C./min by means of a differential scanningcolorimeter and the absolute value of a heat-up crystallization heatamount ΔHc is (|ΔHm|−|ΔHc|)≧25 J/g, a crystallization speed of not lowerthan 0.010 min⁻¹ at 130° C., and a falling ball impact resistance suchthat a falling ball height is not smaller than 20 cm with respect to athickness of 500 μm.

The inventive polylactic acid formed article is preferably formed by oneof vacuum forming, air pressure forming, vacuum air pressure forming andpress forming of the sheet.

A production process for the polylactic acid article formed from thesheet of the resin composition comprising the polylactic acid as themajor resin component according to the second inventive aspect comprisesthe steps of: extruding the resin composition into a sheet, the resincomposition comprising the crystalline polylactic acid resin (A) havingan optical purity of not lower than 95%, the aromatic/aliphaticcopolymer polyester or the aliphatic polyester (B) having a glasstransition temperature of not higher than 0° C., and the talc (C) havingan average particle diameter of 1 to 8 μm with an (A)/(B) blend ratio of(A)/(B)=97/3 to 80/20% by mass and with a (C) blend ratio of 1 to 30% bymass based on the total amount of the composition; heat-treating thesheet at a treatment temperature of 110 to 150° C. for a treatmentperiod of 1 to 30 seconds and forming the sheet into the article.

According to the present invention, it is preferred to form the sheet byone of vacuum forming, air pressure forming, vacuum air pressure formingand press forming after heat-treating the sheet.

According to the present invention, it is preferred to form the sheet byone of vacuum forming, air pressure forming, vacuum air pressure formingand press forming while heat-treating the sheet in a die.

The polylactic acid formed article according to the second inventiveaspect is produced by preparing the sheet by mixing the polylactic acidhaving a strictly adjusted optical purity, the aromatic/aliphaticpolyester or the aliphatic polyester having a glass transitiontemperature of not higher than 0° C. and the talc in the specified blendratio, forming the sheet into the article by a common forming processtypified by the vacuum forming, and heat-treating the sheet under thespecified conditions prior to the forming or heat-treating the sheet inthe die under the predetermined conditions during the forming. Theresulting biodegradable article has thermal properties such that(|ΔHm|−|ΔHc|)≧25 J/g, the crystallization speed is not lower than 0.010min⁻¹, and the falling ball height is not smaller than 20 cm withrespect to a thickness of 500 μm. Thus, the polylactic acid formedarticle has a shock resistance and a heat resistance sufficient towithstand hot water, which cannot be realized in the case of theconventional polylactic acid formed article.

The inventive polylactic acid formed article is applicable to containerssuch as lunch trays, bowls, dishes and cups which are required to haveheat resistance and shock resistance. Since the polylactic acid formedarticle is free from deformation during storage and transportationthereof in summer, the formed article has a variety of applications forlid mateirals, building materials, boards, stationery, cases, carriertapes, cards such as prepaid cards and IC cards and FRPs.

DETAILED DESCRIPTION OF THE INVENTION

The polylactic acid formed article according to the first inventiveaspect should be formed from a sheet of a resin composition comprising aspecified polylactic acid as a major resin component and a specifiedproportion of a crystal nucleus agent.

The polylactic acid formed article according to the second inventiveaspect should be formed from a sheet of a resin composition comprising aspecified polylactic acid, a specified proportion of a specifiedaliphatic/aromatic polyester or a specified aliphatic polyester, and aspecified proportion of a crystal nucleus agent.

A crystalline polylactic acid resin to be employed in the presentinvention should have an optical purity of not lower than 90% (firstaspect) or not lower than 95% (second aspect).

There are two types of monomers of the polylactic acid having differentoptical activities, i.e., D-lactic acid and L-lactic acid. Currently,L-lactic acid is industrially mass-produced at lower costs, andpoly-L-lactic acid (PLLA) derived from L-lactic acid is commonly used.The crystallinity of the polylactic acid varies depending on the contentof L-lactic acid or D-lactic acid. Where the optical purity L of thelactic acid monomers is defined by the following expression (1) forexample, the crystallinity is increased as L (i.e., the optical purity)increases.Optical purity=|M(L)−M(D)|  (1)wherein M(L) is the mole percentage of L-lactic acid units with respectto the total lactic acid units constituting the polylactic acid resin,M(D) is the mole percentage of D-lactic acid units with respect to thetotal lactic acid units constituting the polylactic acid resin, andM(L)+M(D)=100.

More specifically, an optical purity of not lower than 90% according tothe first aspect is equivalent to, for example, a D-isomer content ofnot higher than 5 mol % in the polylactic acid. Examples of thepolylactic acid include poly-L-lactic acid, poly-DL-lactic acid which isa copolymer of L-lactic acid and D-lactic acid, and a mixture of thepoly-L-lactic acid and the poly-DL-lactic acid.

In general, even where the PLLA is prepared, for example, bypolymerizing 100% L-lactic acid monomers thereby to have an opticalpurity of 100%, some of the monomers are racemized by thermal historyexperienced during the polymerization or the subsequent fusion molding.Industrially available PLLA supposedly has an optical purity of about98% at the highest. Hence, this PLLA practically has the highestpossible crystallinity among the polylactic acids. However, even suchPLLA comprising the L-lactic acid monomers with a high purity has arelatively low crystallization speed and, therefore, is highly liable tobe supercooled during a cooling crystallization process.

In order to impart the finally obtained polylactic acid article withheat resistance, it is necessary to promote the crystallization(crystallization speed) of the polylactic acid per se, and to improvethe crystallinity of the polylactic acid after the forming. Therefore,the polylactic acid per se should be able to crystallize to a highcrystallinity. Hence, the polylactic acid resin according to the firstinventive aspect should have an optical purity of not lower than 90%,preferably not lower than 96%. Further, the polylactic acid resinaccording to the second inventive aspect should have an optical purityof not lower than 95%, preferably not lower than 96%. If the opticalpurity of the polylactic acid resin is lower than 90% (first aspect) orlower than 95% (second aspect), the polylactic acid per se has a reducedcrystallinity. Even if talc is added as the crystal nucleus agent or theheat treatment is performed, the polylactic acid is not sufficientlycrystallized, failing to have a desired heat resistance.

Particularly for practical strength and durability, a polymer to beemployed as the polylactic acid preferably has a relatively highmolecular weight, typically a weight-average molecular weight of notsmaller than 100,000, preferably 150,000 to 300,000, more preferably160,000 to 200,000. If the weight-average molecular weight of thepolylactic acid is smaller than 150,000, the melt viscosity is too low,so that the resulting sheet is poor in mechanical characteristics. Ifthe weight-average molecular weight is greater than 300,000, the meltviscosity is too high, resulting in difficulty in fusion extrusion.

It is generally known that, if an excessively great amount of lactide ispresent in the polylactic acid resin, the hydrolysis of the polylacticacid is promoted. However, the lactide which has a lower molecularweight is more easily crystallized than the polylactic acid which has ahigher molecular weight. Therefore, the crystallization of the lactidetriggers and promotes the crystallization of the polylactic acid. Hence,proper setting of the amount of the lactide present in the polylacticacid is effective for the promotion of the crystallization and theimpartation of the heat resistance. That is, the residual lactide amountshould be 0.1 to 0.6% by mass, preferably 0.1 to 0.4% by mass, based onthe total amount of the resin according to the first inventive aspect.According to the second inventive aspect, the residual lactide amountshould be 0.1 to 0.6% by mass, preferably 0.1 to 0.4% by mass, based onthe total amount of the resin. A residual lactide amount of smaller than0.1% by mass is excessively low and, hence, insufficient for triggeringand promoting the crystallization of the polylactic acid. If theresidual lactide amount is greater than 0.6% by mass, the hydrolysis ismore liable to be promoted.

According to the second inventive aspect, the aromatic/aliphaticcopolymer polyester or the aliphatic polyester having a glass transitiontemperature of not higher than 0° C. is essentially employed as acomponent for drastic improvement of the shock resistance as well as theheat resistance of the polylactic acid formed article.

The aromatic/aliphatic copolymer polyester or the aliphatic polyesterhas a glass transition temperature of not higher than 0° C. and, hence,has flexibility at an ordinary temperature. This component is dispersedin the polylactic acid resin, serving to absorb an external shock as ina case where a rubber is dispersed. This contributes to the improvementof the shock resistance. Specific examples of this component include anaromatic/aliphatic copolymer polyester comprising at least an aliphaticdicarboxylic acid, an aromatic dicarboxylic acid and an aliphatic diolas constituents, an aliphatic polyester comprising at least an aliphaticdicarboxylic acid and an aliphatic diol, and an aliphatic polyesterobtained by ring opening polymerization of α-caprolactone as a cyclicmonomer.

Examples of the aliphatic dicarboxylic acid include succinic acid,adipic acid, suberic acid, sebacic acid and dodecanoic diacid. Examplesof the aromatic dicarboxylic acid include terephthalic acid, isophthalicacid and naphthalene dicarboxylic acid. Examples of the aliphatic diolinclude ethylene glycol, propylene glycol, 1,4-butanediol and1,4-cyclohexanedimethanol. The component (B) is obtained bypolycondensation of at least one of the aforesaid components. Asrequired, the component (B) is allowed to have a so-called jump-upstructure and a long chain branch by employing an isocyanate, an acidanhydride, an epoxy compound, an organic peroxide or the like.

In the second inventive aspect, where the crystalline polylactic acidresin having an optical purity of not lower than 95% is defined as thecomponent (A) and the aromatic/aliphatic copolymer polyester or thealiphatic polyester having a glass transition temperature of not higherthan 0° C. is defined as the component (B), the blend ratio of thecomponent (A) and the component (B) should be (A)/(B)=97/3 to 80/20% bymass, preferably (A)/(B)=97/3 to 85/15, more preferably (A)/(B)=95/5 to85/15% by mass. If the blend ratio of the component (B) is smaller than3% by mass, the external shock cannot be absorbed, resulting in a lowershock resistance. On the other hand, if the blend ratio of the component(B) is greater than 20% by mass, the shock resistance is remarkablyimproved. However, the crystallization of the polylactic acid per se ishindered, resulting in a lower heat resistance. In addition, thecrystallization speed is reduced, so that the production requires aprolonged forming cycle time to reduce the productivity.

In the first inventive aspect, the resin composition may furthercomprise another resin component such as an aliphatic polyester, analiphatic/aromatic copolymer polyester or a polyester carbonate, as faras the properties of the polylactic acid are not deteriorated.

In the first inventive aspect, the crystal nucleus agent should becontained in an amount of 1 to 25% by mass in the resin composition. Ifthe amount of the crystal nucleus agent is smaller than 1% by mass, theeffect of the crystal nucleus agent is insufficient. If the amount ofthe crystal nucleus agent is greater than 25% by mass, the excessivelygreat amount of the crystal nucleus agent adversely influences theproperties of the resin composition, for example, the resulting formedarticle is embrittled. Therefore, the amount of the crystal nucleusagent in the resin composition is preferably in a range of 1 to 20% bymass, more preferably 1 to 15% by mass.

The crystal nucleus agent preferably has an average particle diameter of0.1 to 10 μm. If the average particle diameter is smaller than 0.1 μm,the effect of the crystal nucleus agent is insufficient withinsufficient dispersion and secondary coagulation. If the averageparticle diameter is greater than 10 μm, the properties of the sheet areadversely influenced, so that the properties of the formed article areadversely influenced.

The crystal nucleus agent is not particularly limited, but examples ofthe crystal nucleus agent include laminar silicates typified by talc,smectite, vermiculite and swellable fluorine-containing mica, amongwhich the talc is preferably used as the crystal nucleus agent becauseit is an inorganic substance having the highest crystallizationefficiency for the polylactic acid. Further, the talc is preferredbecause it is a very cheap and naturally existing inorganic substancewhich is industrially advantageous and imposes no burden to the globalenvironment.

In the second aspect of the present invention, it is essential tooptimize the polylactic acid resin per se as described above and toblend the talc as the crystal nucleus agent for the promotion of thecrystallization.

The talc as the crystal nucleus agent in the second inventive aspect hasan average particle diameter of 1 to 8 μm, preferably 1 to 5 μm. Amongthe various crystal nucleus agents, the talc which is an inorganicsubstance having the highest crystallization efficiency for thepolylactic acid is the most preferred crystal nucleus agent. Further,the talc is industrially advantageous and imposes no burden to thenatural environment, because it is a very cheep and naturally existinginorganic substance. If the average particle diameter of the talc issmaller than 1 μm, the effect of the crystal nucleus agent isinsufficient with insufficient dispersion and secondary coagulation.Therefore, the resulting formed article has an insufficient heatresistance. If the average particle diameter is greater than 8 μm, thetalc serving as the crystal nucleus agent causes defect in the formedarticle, thereby adversely influencing the properties and surface stateof the formed article.

The content of the talc is 1 to 30% by mass, preferably 5 to 20% bymass, more preferably 10 to 15% by mass, based on the total amount ofthe composition. A content of smaller than 1% by mass is excessively lowand, therefore, only a small amount of crystal nuclei are generated, sothat the effect of the crystal nucleus agent is insufficient. Hence, theresulting formed article has an insufficient heat resistance. A contentof greater than 30% by mass is excessively high, so that the propertiesof the resulting formed article is adversely influenced, for example,the formed article is embrittled.

A dispersant may be employed for efficiently dispersing the crystalnucleus agent in the resin composition for the sheet. The dispersant ispreferably highly compatible with the polylactic acid and has anexcellent wettability with respect to the crystal nucleus agent. It isimportant to select at least one of aliphatic amides includingerucamide, stearamide, oleamide, ethylene-bis-stearamide,ethylene-bis-oleamide and ethylene-bis-laurylamide as the dispersant forefficiently increasing the crystallinity of the polylactic acid formedarticle.

In the present invention, a cross-linking agent such as an organicperoxide and an auxiliary cross-linking agent may be employed incombination as required for lightly cross-linking the resin compositionfor the acceleration of the crystallization speed by the crystal nucleusagent.

Specific examples of the cross-linking agent include: organic peroxidessuch as n-butyl-4,4-bis-t-butyl peroxyvalerate, dicumyl peroxide,di-t-butyl peroxide, di-t-hexyl peroxide,2,5-dimethyl-2,5-di(t-butylperoxy)hexane and2,5-dimethyl-2,5-t-butylperoxyhexyne-3; polycarboxylic acids such asphthalic anhydride, maleic anhydride, trimethyl adipate, trimelliticanhydride and 1,2,3,4-butanetetracarboxylic acid; metal complexes suchas lithium formate, sodium methoxide, potassium propionate and magnesiumethoxide; epoxy compounds such as bisphenol-A diglycidyl ether,1,6-hexanediol diglycidyl ether, trimethylolpropane triglycidyl etherand diglycidyl terephthalate; and isocyanate compounds such asdiisocyanates, triisocyanates, hexamethylene diisocyanate, 2,4-tolylenediisocyanate, 2,6-tolylene diisocyanate, xylylene diisocyanate anddiphenylmethane diisocyanate.

Examples of the auxiliary cross-linking agent include trimethacrylates,glycidyl methacrylate, n-butyl methacrylate, hydroxypropylmonomethacrylate and polyethylene glycol monomethacrylate.

The polylactic acid formed article according to the first inventiveaspect should be formed from the sheet of the resin compositioncomprising the specified polylactic acid as the major resin componentand the specified proportion of the crystal nucleus agent as describedabove. The polylactic acid formed article according to the secondinventive aspect should be formed from the sheet of the resincomposition comprising the specified proportion of the specifiedpolylactic acid, the specified proportion of the specifiedaliphatic/aromatic polyester or the specified aliphatic polyester andthe specified proportion of the crystal nucleus agent as describedabove.

The formed articles according to the inventive aspects should each havea crystallization index such that a difference between the absolutevalue of a crystal fusion heat amount ΔHm as measured at a heat-up rateof 20° C./min by means of a differential scanning calorimeter and theabsolute value of a heat-up crystallization heat amount ΔHc is(|ΔHm|−|ΔHc|)≧25 J/g, preferably (|ΔHm|−|ΔHc|)≧29 J/g. To satisfy theexpression (|ΔHm|−|ΔHc|)≧25 J/g, it is necessary to optimize the opticalpurity and the residual-lactide amount of the polylactic acid to beemployed and the average particle diameter and the addition amount ofthe crystal nucleus agent (talc), and to perform a heat treatment to bedescribed later.

If (|ΔHm|−|ΔHc|)<25 J/g, the article is not sufficiently crystallized.When hot water (90° C.) is poured in a container formed from an ordinarypolylactic acid, for example, the container is liable to be thermallydeformed with an insufficient heat resistance. However, the formedarticle is free from such a phenomenon, if (|ΔHm|−|ΔHc|)≧25 J/g.

The polylactic acid formed article according to the first inventiveaspect should satisfy the expression (|ΔHm|−|ΔHc|)≧25 J/g as describedabove, and have a crystallinity of not lower than 35% as determined byX-ray measurement and a crystallization speed of not lower than 0.05min⁻¹ at 130° C.

In order that the crystallinity determined by the X-ray measurement canbe set at not lower than 35%, it is necessary to optimize the opticalpurity and the residual lactide amount of the polylactic acid to beemployed and the average particle diameter and the addition amount ofthe crystal nucleus agent (talc), and to perform a heat treatment to bedescribed later.

In order that the crystallization speed at 130° C. can be set at notlower than 0.05 min⁻¹, it is necessary to employ the crystallinepolylactic acid having an optical purity of not lower than 90% and aresidual lactide amount of 0.1 to 0.6% by mass, to blend the crystalnucleus agent having an average particle diameter of 0.1 to 10 μm in aproportion of 1 to 30% by mass based on the total amount of thecomposition and prepare the sheet from the resin composition, and toheat-treat the sheet at a temperature of 110 to 150° C. for 1 to 30seconds.

According to the first aspect, only if all the aforesaid threeconditions are satisfied, the polylactic acid is supposedly crystallizedto an extent sufficient to impart the formed article with the heatresistance according to the first aspect. Therefore, if at least one ofthe conditions is not satisfied in the first aspect, the polylactic acidis insufficiently crystallized and, hence, the resulting formed articlehas a lower heat resistance.

In the second inventive aspect, the heat treatment is essential in theproduction of the formed article. However, it is industrially impossibleto perform the heat treatment for a long period of time. On the otherhand, it is known that the polylactic acid has an extremely lowcrystallization speed. Therefore, it is necessary to impart thepolylactic acid with a crystallization speed industrially suitable forthe forming cycle. In the second aspect, the industrial production ofthe desired formed article can be achieved by precisely optimizing thecomposition of the polylactic acid, the crystal nucleus agent and theheat treatment conditions. The formed article according to the secondaspect should have a crystallization speed of not lower than 0.010min⁻¹, preferably not lower than 0.015 min⁻¹, at 130° C. Acrystallization speed of lower than 0.010 min⁻¹ at 130° C. is too lowand unsuitable for the ordinary forming cycle, leading to insufficientcrystallization and hence a lower heat resistance. In order that thecrystallization speed at 130° C. can be set at not lower than 0.010min⁻¹ according to the second aspect, it is necessary to optimize theoptical purity of the polylactic acid to be employed, to optimize theaverage particle diameter and the blend ratio of the talc, to optimizethe blend ratio between the aromatic/aliphatic copolymer polyester orthe aliphatic polyester and the polylactic acid, and to perform the heattreatment at a treatment temperature of 110 to 150° C. for a treatmentperiod of 1 to 30 seconds as will be described later.

The polylactic acid formed article according to the second inventiveaspect should have a falling ball impact resistance such that a fallingball height is not smaller than 20 cm with respect to a thickness of 500μm. The falling ball height with respect to a thickness of 500 μm isherein determined by horizontally placing a box-shaped article formedfrom a 500 μm thick sheet with its bottom upward, and dropping a 300 giron ball from different heights in increments of 5 cm onto the articlea plurality of times, and defined as [(a height at which the article isbroken at a 50% probability)−5] cm.

If the falling ball height with respect to a thickness of 500 μm issmaller than 20 cm, the formed article is liable to be broken or crackedwhen an external shock is applied to the formed article duringtransportation. Therefore, the falling ball height with respect to athickness of 500 μm is preferably not smaller than 30 cm.

In order to impart the formed article with a falling impact resistancesuch that the falling ball height with respect to a thickness of 500 μmis not smaller than 20 cm, the aromatic/aliphatic copolymer polyester orthe aliphatic polyester having a glass transition temperature of nothigher than 0° C. should be blended in a proportion of not smaller than3% by mass in the crystalline polylactic acid resin having an opticalpurity of not lower than 95%.

Next, an explanation will be given to the molecular weight retentionratio of the formed article according to the present invention. Themolecular weight retention ratio is herein a percentage obtained bydividing a weight-average molecular weight (Mw) determined after adecomposition acceleration test in which the formed article is allowedto stand in a constant-temperature constant-humidity chamber under 50°C./90% RH conditions for 30 days by a weight-average molecular weightdetermined before the decomposition acceleration test. In general,biodegradable polylactic acid resin products (sheets and formedarticles) are preferably insusceptible to decomposition during thestorage or use thereof, but are decomposed speedily after use.Therefore, the molecular weight retention ratio under the aforesaidconditions of the decomposition acceleration test is preferably notlower than 60%, more preferably not lower than 70%. A molecular weightretention ratio of lower than 60% means that the decomposition speed ishigh and, hence, is disadvantageous in that the resulting formed articleis liable to be decomposed during the storage thereof and is unsuitablefor practical use.

As required, a plasticizer, a UV protective agent, a light stabilizer,an anti-hazing agent, an anti-misting agent, antistatic agent, a flameretarder, an anti-coloring agent, an antioxidant, a filler and a pigmentmay be added to the resin composition, as far as the properties of theresin composition are not deteriorated.

Next, an explanation will be given to a production process for thepolylactic acid formed article according to the first inventive aspect.

The polylactic acid resin having an optical purity of not lower than 90%and a residual lactide amount of 0.1 to 0.6% by mass and the crystalnucleus agent and, as required, the dispersant are first mixed together,and the resulting mixture was melted to be formed into a sheet. Themethod for the preparation of the sheet is not particularly limited, buta T-die method, an inflation method or a calender method, for example,may be employed for the preparation of the sheet. Particularly, theT-die method is preferred, in which a T-die is employed formelt-kneading and extrusion. Where the preparation is achieved by theT-die, an unstretched sheet having a thickness of about 150 to 500 μm isprepared, for example, by supplying the ingredients into a hopper of auniaxial extruder or a biaxial extruder, heating the extruder at acylinder temperature of 180 to 230° C. and at a T-die temperature of 200to 230° C., melting and kneading the ingredients and extruding theresulting mixture into a sheet, and cooling the sheet on a cast rolladjusted at a temperature of 30 to 50° C. The thickness of theunstretched sheet is not particularly limited, but may properly bedetermined according to the application, the property requirements, thecost and the like.

The article is formed from the resulting sheet by a forming process. Inorder to impart the formed article with the aforesaid required thermalcharacteristics, it is necessary to employ the polylactic acid resincomposition having the specified formulation, and to improve thecrystallinity of the polylactic acid in the formed article by performingthe heat treatment under specified conditions before the sheet formingor during the sheet forming.

More specifically, the temperature for the heat treatment should be setat a temperature of 110 to 150° C. at which the polylactic acid ispractically most easily crystallized. If the heat treatment temperatureis lower than 110° C., the polylactic acid is insufficientlycrystallized. If the heat treatment temperature is higher than 150° C.,the crystallization speed of the polylactic acid is extremely low.Further, crystals of the polylactic acid are melted thereby to result ininsufficient crystallization, because the temperature is close to themelting point of the polylactic acid. Therefore, the heat treatmenttemperature is preferably in a range of 125 to 150° C., particularlypreferably 125 to 145° C.

Further, the heat treatment period should be in a range of 1 to 30seconds, which is practically applicable to the production cycle andjust enough for the crystallization. A heat treatment period of shorterthan 1 second is insufficient for the crystallization of the polylacticacid. A heat treatment period of longer than 30 seconds is notapplicable to the practical production cycle, and is industriallydisadvantageous. Therefore, the heat treatment period is preferably in arange of 3 to 30 seconds, particularly preferably 3 to 20 seconds.

The method for the forming of the sheet is not particularly limited, butone of vacuum forming, air pressure forming, vacuum air pressure formingand press forming is preferably employed for the forming.

In the production process for the polylactic acid formed article, theformed article which is excellent in heat resistance with thermalcharacteristics such that a difference between the absolute value of thecrystal fusion heat amount ΔHm measured at a heat-up rate of 20° C./minby means of the differential scanning calorimeter and the absolute valueof the heat-up crystallization heat amount ΔHc is not smaller than 25J/g, the crystallinity determined by the X-ray measurement is not lowerthan 35%, and the crystallization speed at 130° C. is not lower than0.05 min⁻¹ according to the first inventive aspect can industrially beproduced in a practical production cycle.

Next, an explanation will be given to a production process for thepolylactic acid formed article according to the second inventive aspect.

The polylactic resin (A) having an optical purity of not lower than 95%,the aromatic/aliphatic copolymer polyester or the aliphatic polyester(B) having a glass transition temperature of not higher than 0° C. andthe talc (C) and, as required, the dispersant are blended in thespecified ratio. In this case, all the components are preliminarilycompounded in a biaxial extruder. Alternatively, only the components (A)and (C) may be compounded, and then dry-blended with the compound (B),or all the components may be dry-blended. Thereafter, the sheet isprepared by melting and kneading the resulting resin composition in auniaxial extruder or a biaxial extruder having a T-die and extruding theresin composition through the T-die onto a cast roll adjusted at atemperature of 30 to 50° C. The thickness of the sheet may properly beselected according to the application. The thickness of the sheet ispreferably 200 to 750 μm.

Thereafter, the unstretched sheet is heat-treated under the followingconditions in a continuous or separate step, and then formed into thetarget article by any one of press forming, vacuum forming, air pressureforming and vacuum air pressure forming. Alternatively, the unstretchedsheet may be formed into the article by any of the forming methods,while being heat-treated in a die.

In the present invention, the resin, the crystal nucleus agent and thelike should precisely be optimized, and the heat treatment should beperformed at a treatment temperature of 110 to 150° C. for a treatmentperiod of 1 to 30 seconds. At a treatment temperature of 110 to 150° C.,the polylactic acid is practically most easily crystallized. A treatmentperiod of 1 to 30 seconds is practically applicable to the productioncycle and just enough for the crystallization. If the treatmenttemperature is lower than 110° C., the polylactic acid is insufficientlycrystallized. On the other hand, if the heat treatment temperature ishigher than 150° C., the crystallization speed is extremely low,resulting in insufficient crystallization. A treatment period of shorterthan 1 second is insufficient for the crystallization. A treatmentperiod of longer than 30 seconds is not applicable to the practicalproduction cycle, and is industrially disadvantageous.

EXAMPLES

Next, an explanation will be given to examples and comparative examples.

In the following examples and comparative examples, the characteristicproperties are determined in the following manner.

(1) Crystal Fusion Heat Amount ΔHm and Heat-Up Crystallization HeatAmount ΔHc

When a 10 mg test sample of a formed article was heated up at a heat-uprate of 20° C./min with the use of Perkin Elmer's Pyrisl DSC, a totalheat amount at a peak appearing on an exothermic side was determined asthe heat-up crystallization heat amount ΔHc, and a total heat amount ata peak appearing on an endothermic side was determined as a crystalfusion heat amount ΔHm.

(2) Crystallinity Determined by X-Ray Measurement

The formed article to be subjected to measurement was powdered, and themeasurement was performed by a WAXD reflection powder method with theuse of an X-ray diffractometer (RAD-rB available from Rigaku DenkiKogyo). Then, an integrated intensity ratio was determined by amulti-peak separation method.

(3) Crystallization Speed

With the use of Perkin Elmer's Pyrisl DSC, a test sample was heated from20° C. to 200° C. at 500° C./min, then kept at 200° C. for 5 minutes,and cooled to 130° C. at −500° C./min. Then, the measurement wascontinued until the completion of the crystallization. Thereafter, thecrystallization speed was determined by multiplying an inversion of aperiod required until a crystallization ratio of 0.5 was reached by acrystallization ratio of 0.5.

(4) Heat Resistance

With the use of a single-shot indirect-heating vacuum forming machineand an aluminum die CT-DELICAN 15-11, a container having a length of 150mm, a width of 110 mm and a depth of 20 mm was formed from a sheet.Then, 90° C. hot water was poured in the container and, after 5 minutes,the container was visually observed to check for deformation thereof.Where no deformation was observed, the container was regarded to have anexcellent heat resistance and indicated by ◯ in evaluation. Where slightdeformation was observed, the container was regarded to have a slightlylower heat resistance and indicated by Δ in evaluation. Where remarkabledeformation was observed, the container was regarded to have aninsufficient heat resistance and indicated by x in evaluation.

(5) Molecular Weight Retention Ratio

The weight-average molecular weight (Mw) of polylactic acid after a testsample was allowed to stand in a constant-temperature constant-humiditychamber under 50° C./90% RH conditions for 30 days was determined bypassing a THF solution containing the polylactic acid through a StyragelHR column and an Ultrastyragel column with the use of polystyrene as areference substance by means of a refractometer as a detector. Theretention ratio was calculated from the following expression:Mw retention ratio (%)=(Mw after 30−day test/Mw before test)×100

The molecular weight retention ratio is regarded as the index ofhydrolysis. It is supposed that the extent of the hydrolysis isincreased as the molecular weight retention ratio decreases.

(6) Shock Resistance

A box-shaped article formed from a 500 μm thick sheet and subjected tothe heat treatment was horizontally placed with its bottom upward, and a300 g iron ball was dropped onto the article from different heights inincrements of 5 cm. The shock resistance was evaluated by a falling ballheight defined as [(a height at which the article was broken at a 50%probability)−5] cm.

Example 1

First, 84% by mass of polylactic acid (NATURE WORKS available fromCargill Dow) having an optical purity of 97.6%, a residual lactideamount of 0.2% by mass and a weight-average molecular weight of 200,000,15% by mass of talc as a crystal nucleus agent (MW HS-T available fromHayashi Kasei) having an average particle diameter of 2.75 μm and 1% bymass of erucamide (ALFLOW P10 available from Nihon yushi) as adispersant were melt-kneaded by means of a biaxial kneader/extruder(MODEL TEX44α available from Nihon Seikosho) to prepare a polylacticacid compound material at an extrusion temperature of 230° C. With theuse of a uniaxial extruder having a 1000-mm wide T-die and a screwhaving a diameter of 90 mm, the polylactic acid compound material wasmelt-extruded at an extrusion temperature of 215° C. to provide a 350 μmthick unstretched sheet in intimate contact with a cast roll set at 40°C. With the use of a single-shot indirect-heating vacuum forming machineand an aluminum die (CT DELICAN 15-11), a container (formed article)having a length of 150 mm, a width of 110 mm and a depth of 20 mm wasformed from the sheet by vacuum forming. A heat treatment was performedat a die inside temperature of 140° C. for 5 seconds during the vacuumforming.

The properties of the resulting formed article and the like are shown inTable 1. Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Example 7 Polylactic Optical purity (%) 97.6 92.0 97.8 97.6 97.6 97.697.6 Acid Residual lactide 0.2 0.2 0.4 0.2 0.2 0.2 0.2 (mass %) TalcContent (mass %) 15 15 15 1 15 15 15 Average particle 2.75 2.75 2.752.75 2.75 2.75 2.75 diameter (μm) Heat treatment temperature 140 125 140140 145 130 140 (° C.) Heat treatment period (sec) 5 15 5 5 3 20 7 |ΔHm|− |ΔHc| 33.7 31.7 36.0 26.0 29.5 27.0 34.0 Crystallinity (%) 42 40 43 3538 36 43 Crystallization speed (min⁻¹) 0.085 0.055 0.095 0.100 0.0850.085 0.085 Heat resistance ◯ ◯ ◯ ◯ ◯ ◯ ◯ Molecular weight 82 80 85 8578 75 82 retention ratio (%) Comparative Comparative ComparativeComparative Comparative Example 1 Example 2 Example 3 Example 4 Example5 Polylactic Optical purity (%) 97.6 97.6 80.0 97.6 97.6 acid Residuallactide 0.2 0.2 0.2 1.0 0.2 (mass %) Talc Content (mass %) 0 40 15 15 15Average particle — 2.75 2.75 2.75 2.75 diameter (μm) Heat treatmenttemperature 140 140 140 140 110 (° C.) Heat treatment period (sec) 5 5 55 60 |ΔHm| − |ΔHc| 8.2 21.3 0 35.5 11.2 Crystallinity (%) 4 29 0 43 15Crystallization speed (min⁻¹) 0.005 0.040 0 0.100 0.085 Heat resistanceX Δ X ◯ X Molecular weight retention 35 80 25 10 35 ratio (%)

Example 2

An unstretched sheet was prepared and a container (formed article) wasformed from the sheet in substantially the same manner as in Example 1,except that polylactic acid (NATURE WORKS available from Cargill Dow)having an optical purity of 92.0%, a residual lactide amount of 0.2% bymass and a weight-average molecular weight of 190,000 was employed. Theheat treatment was performed at a die inside temperature of 125° C. for15 seconds during the vacuum forming.

The properties of the resulting formed article and the like are shown inTable 1.

Example 3

A container (formed article) was produced in substantially the samemanner as in Example 1, except that polylactic acid (NATURE WORKSavailable from Cargill Dow) having an optical purity of 97.8%, aresidual lactide amount of 0.4% by mass and a weight-average molecularweight of 200,000 was employed.

The properties of the resulting formed article and the like are shown inTable 1.

Example 4

A container (formed article) was produced in substantially the samemanner as in Example 1, except that the content of the talc as thecrystal nucleus agent was 1% by mass.

The properties of the resulting formed article and the like are shown inTable 1.

Examples 5 and 6

Containers (formed articles) were produced in substantially the samemanner as in Example 1, except that the die inside temperature and theheat treatment period for the heat treatment performed simultaneouslywith the vacuum forming were as shown in Table 1.

The properties of the resulting formed articles and the like are shownin Table 1.

Example 7

A container (formed article) was produced in substantially the samemanner as in Example 1, except that the unstretched sheet waspreliminarily heat-treated at 140° C. for 7 seconds and the vacuumforming was performed at a die inside temperature of 125° C. for atreatment period of 1 second after the heat treatment.

The properties of the resulting formed article and the like are shown inTable 1.

The containers (formed articles) of Examples 1 to 6 were each producedby preparing a sheet of a resin composition containing polylactic acidhaving a D-lactic acid content and a residual lactide amount within theranges specified by the present invention and the crystal nucleus agentin a proportion within the range specified by the present invention, andheat-treating the sheet at a temperature for a period within the rangesspecified by the present invention simultaneously with the forming ofthe sheet. Therefore, the formed articles each had a high crystallinity,and were excellent in heat resistance. The container (formed article) ofExample 7 was produced by performing the vacuum forming after the heattreatment of the sheet at a temperature for a period within the rangesspecified by the present invention rather than simultaneously with theheat treatment. Therefore, the formed article had a high crystallinity,and was excellent in heat resistance.

Comparative Example 1

A container (formed article) was produced in substantially the samemanner as in Example 1, except that the talc was not added as thecrystal nucleus agent.

The properties of the resulting container and the like are shown inTable 1.

Comparative Example 2

A container (formed article) was produced in substantially the samemanner as in Example 1, except that the talc was added as the crystalnucleus agent in an amount of 40% by mass which was greater than therange specified by the present invention.

The properties of the resulting container and the like are shown inTable 1.

Comparative Example 3

A container (formed article) was produced in substantially the samemanner as in Example 1, except that polylactic acid (NATURE WORKSavailable from Cargill Dow) having an optical purity of 80.0% which waslower than the range specified by the present invention, a residuallactide amount of 0.2% by mass and a weight-average molecular weight of200,000 was employed.

The properties of the resulting container and the like are shown inTable 1.

Comparative Example 4

A container (formed article) was produced in substantially the samemanner as in Example 1, except that polylactic acid (NATURE WORKSavailable from Cargill Dow) having an optical purity of 97.6%, aresidual lactide amount of 1.0% by mass which was greater than the rangespecified by the present invention and a weight-average molecular weightof 200,000 was employed.

The properties of the resulting container and the like are shown inTable 1.

Comparative Example 5

A container (formed article) was produced in substantially the samemanner as in Example 1, except that the heat treatment temperature was110° C. which was lower than the range specified by the presentinvention and the heat treatment period was 60 seconds which was longerthan the range specified by the present invention.

The properties of the resulting container and the like are shown inTable 1.

In Comparative Example 1, the crystal nucleus agent was not added to theresin composition for the sheet and, hence, it was impossible toaccelerate the crystallization speed, resulting in a lower productivity.Further, the crystallization was insufficient. Therefore, when hot waterwas poured in the resulting container, the container was deformed in aninstance with a poor heat resistance.

In Comparative Example 2, the crystal nucleus agent was added in anexcessively great amount, so that the prepared sheet was embrittled.Therefore, the sheet was easily cracked during the forming, and crackswere observed in the resulting container. Further, the container did nothave a heat resistance sufficient for practical use.

In Comparative Example 3, the optical purity of the polylactic acid waslower than the range specified by the present invention, so that thepolylactic acid had a reduced crystallinity. Even though the crystalnucleus agent was added and the heat treatment conditions were properlyadjusted, it was impossible to promote the crystallization of thepolylactic acid. It was possible to produce the formed article, but thearticle was stuck to the die, resulting in a reduced productivity.Further, the crystallinity was insufficient, so that the container waspoor in heat resistance.

In Comparative Example 4, the residual lactide amount of the polylacticacid was greater than the range specified by the present invention. Eventhough the crystallization was promoted, hydrolysis and thermaldecomposition were also promoted by lactide as apparent from a lowermolecular weight retention ratio. Therefore, the formed article was verybrittle, and problematic in practical use.

In Comparative Example 5, the heat treatment temperature was lower thanthe range specified by the present invention. Even though the heattreatment period was longer than the range specified by the presentinvention, the crystallization was insufficient and the heat resistancewas poorer.

Example 8

Crystalline polylactic acid (A) (NATURE WORKS available from CargillDow, and having an optical purity of 97.2%, a residual lactide amount of0.2% by mass and a weight-average molecular weight of 200,000) and anaromatic/aliphatic copolymer polyester (B) (ECOFLEX F available fromBASF and having a glass transition temperature of −30° C.) having aglass transition temperature of not higher than 0° C. were blended in aratio of (A)/(B)=90/10% by mass, and then talc (MW HS-T available fromHayashi Kasei) having an average particle diameter of 2.75 μm was addedto the resulting blend in an amount of 10% by mass based on the totalamount of the composition. The resulting blend was melt-kneaded by meansof a biaxial kneader/extruder (MODEL TEX44α available from NihonSeikosho) to prepare a polylactic acid compound material at an extrusiontemperature of 230° C.

With the use of a uniaxial extruder having a 1000 mm wide T-die and ascrew having a diameter of 90 mm, the polylactic acid compound materialwas melt-extruded at an extrusion temperature of 215° C. to provide anunstretched sheet having a thickness of 500 μm on a cast roll set at 40°C.

With the use of a single-shot indirect-heating vacuum forming machineand an aluminum die CT DELICAN 15-11, a container (formed article)having a length of 150 mm, a width of 110 mm and a depth of 20 mm wasformed from the sheet by vacuum forming. In the vacuum forming, a heattreatment was performed at a die inside temperature of 140° C. for aretention period of 5 seconds.

The properties of the resulting formed article are shown in Table 2.TABLE 2 Example 8 9 10 11 12 13 14 15 16 Polylactic Optical purity (%)97.2 96.0 97.2 97.2 97.2 97.2 97.2 97.2 97.2 acid (A) Residual lactideamount 0.2 0.4 0.2 0.2 0.2 0.2 0.2 0.2 0.2 (mass %) B1 or Type B1 B1 B1B1 B1 B2 B1 B1 B1 B2 (B) Glass transition −30 −30 −30 −30 −30 −30 −30−30 −30 temperature (° C.) (A)/(B) (mass %) 90/10 90/10 85/15 90/1090/10 90/10 90/10 95/5 90/10 Talc (C) Content (mass %) 10 10 10 15 10 1010 10 10 Average particle 2.75 2.75 2.75 2.75 4.1 2.75 2.75 2.75 2.75diameter (μm) Heat Temperature (° C.) 140 120 140 140 140 140 150 130140 treatment Period (sec) 5 15 5 5 5 5 3 20 10 |ΔHm| − |ΔHc| (J/g) 31.529.0 29.0 33.5 30.0 31.7 27.2 29.5 29.5 Crystallization speed (min⁻¹)0.018 0.016 0.015 0.033 0.017 0.020 0.018 0.030 0.018 Heat resistance ◯◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ Molecular weight retention ratio (%) 80 75 78 82 78 8070 78 82 Falling ball height (cm) 50 50 90 50 50 50 50 30 50 ComparativeExample 6 7 8 9 10 11 12 Polylactic Optical purity (%) 97.2 97.2 80.097.2 97.2 97.2 97.2 acid (A) Residual lactide amount 0.2 0.2 0.5 0.2 0.20.2 0.2 (mass %) B1 or Type B1 B1 B1 None B1 B1 B1 B2 (B) Glasstransition −30 −30 −30 −30 −30 −30 temperature (° C.) (A)/(B) (mass %)90/10 90/10 90/10 100/0 70/30 90/10 90/10 Talc (C) Content (mass %) 0 4010 10 10 10 10 Average particle — 2.75 2.75 2.75 2.75 2.75 2.75 diameter(μm) Heat Temperature (° C.) 140 140 140 140 140 160 100 treatmentPeriod (sec) 5 5 5 5 5 5 60 |ΔHm| − |ΔHc| (J/g) 6.5 19.8 0 32.0 16.510.0 9.0 Crystallization speed (min⁻¹) 0.001 0.035 0 0.070 0.006 0.0180.018 Heat resistance x x x ◯ x x x Molecular weight retention ratio (%)30 50 25 80 40 15 15 Falling ball height (cm) 95 10 25 5 ≧100 90 90B1: Aromatic/aliphatic copolymer polyesterB2: Aliphatic polyester

Example 9

An unstretched sheet was prepared and a container (formed article) wasformed from the sheet by vacuum forming in substantially the same manneras in Example 8, except that polylactic acid (NATURE WORKS availablefrom Cargill Dow, and having an optical purity of 96.0%, a residuallactide amount of 0.4% by mass and a weight-average molecular weight of190,000) was employed as the crystalline polylactic acid (A). In thevacuum forming, a heat treatment was performed at a die insidetemperature of 120° C. for a retention period of 15 seconds.

The properties of the resulting formed article are shown in Table 2.

Example 10

An unstretched sheet was prepared and a container (formed article) wasformed from the sheet by vacuum forming in substantially the same manneras in Example 8, except that the ratio between the crystallinepolylactic acid (A) and the aromatic/aliphatic copolymer polyester (B)having a glass transition temperature of not higher than 0° C. was(A)/(B)=85/15% by mass.

The properties of the resulting formed article are shown in Table 2.

Example 11

An unstretched sheet was prepared and a container (formed article) wasformed from the sheet by vacuum forming in substantially the same manneras in Example 8, except that the talc (C) was added in an amount of 15%by mass based on the total amount of the composition.

The properties of the resulting formed article are shown in Table 2.

Example 12

An unstretched sheet was prepared and a container (formed article) wasformed from the sheet by vacuum forming in substantially the same manneras in Example 8, except that talc (C) (MICRON WHITE #5000A availablefrom Hayashi Kasei) having an average particle diameter of 4.1 μm wasemployed.

The properties of the resulting formed article are shown in Table 2.

Example 13

An unstretched sheet was prepared and a container (formed article) wasformed from the sheet by vacuum forming in substantially the same manneras in Example 8, except that an aliphatic polyester (B) (BIONOLE 3001available from Showa Polymer and having a glass transition temperatureof −30° C.) having a glass transition temperature of not higher than 0°C. was employed.

The properties of the resulting formed article are shown in Table 2.

Example 14

An unstretched sheet was prepared and a container (formed article) wasformed from the sheet by vacuum forming in substantially the same manneras in Example 8, except that the conditions for the heat treatment inthe die were changed to a temperature of 150° C. and a retention periodof 3 seconds as shown in Table 2.

The properties of the resulting formed article are shown in Table 2.

Example 15

An unstretched sheet was prepared and a container (formed article) wasformed from the sheet by vacuum forming in substantially the same manneras in Example 8, except that the ratio between the crystallinepolylactic acid (A) and the aromatic/aliphatic copolymer polyester (B)having a glass transition temperature of not higher than 0° C. was(A)/(B)=95/5% by mass and the conditions for the heat treatment in thedie were changed to a temperature of 130° C. and a retention period of20 seconds as shown in Table 2.

The properties of the resulting formed article are shown in Table 2.

Example 16

An unstretched sheet prepared in the same manner as in Example 8 wassubjected to a heat treatment at 140° C. for 10 seconds. Thereafter, acontainer (formed article) having a length of 150 mm, a width of 110 mmand a depth of 20 mm was formed from the sheet by vacuum forming withthe use of a single-shot indirect-heating vacuum forming machine and analuminum die CT DELICAN 15-11. In the vacuum forming, the die insidetemperature was 125° C. and the forming cycle was 1 second.

The properties of the resulting formed article are shown in Table 2.

Comparative Example 6

An unstretched sheet was prepared and a container (formed article) wasformed from the sheet by vacuum forming in substantially the same manneras in Example 8, except that the talc was not employed.

The properties of the resulting formed article are shown in Table 2.

Comparative Example 7

An unstretched sheet was prepared and a container (formed article) wasformed from the sheet by vacuum forming in substantially the same manneras in Example 8, except that the content of the talc was 40% by mass.

The properties of the resulting formed article are shown in Table 2.

Comparative Example 8

An unstretched sheet was prepared and a container (formed article) wasformed from the sheet by vacuum forming in substantially the same manneras in Example 8, except that polylactic acid (NATURE WORKS availablefrom Cargill Dow, and having an optical purity of 80.0%, a residuallactide amount of 0.5% by mass and a weight-average molecular weight of200,000) was employed as the polylactic acid (A).

The properties of the resulting formed article are shown in Table 2.

Comparative Example 9

An unstretched sheet was prepared and a container (formed article) wasformed from the sheet by vacuum forming in substantially the same manneras in Example 8, except that the polyester (B) having a glass transitiontemperature of not higher than 0° C. was not employed but onlycrystalline polylactic acid (A) and talc (C) as employed in Example 8were employed.

The properties of the resulting formed article are shown in Table 2.

Comparative Example 10

An unstretched sheet was prepared and a container (formed article) wasformed from the sheet by vacuum forming in substantially the same manneras in Example 8, except that the blend ratio between the crystallinepolylactic acid (A) and the aromatic/aliphatic copolymer polyester (B)having a glass transition temperature of not higher than 0° C. was(A)/(B)=70/30% by mass.

The properties of the resulting formed article are shown in Table 2.

Comparative Example 11

An unstretched sheet was prepared in the same manner as in Example 8,and a container (formed article) was formed from the sheet insubstantially the same manner as in Example 8 by means of a formingmachine as employed in Example 8, except that the heat treatmentconditions were changed to perform the heat treatment in a die at 160°C. for 5 seconds.

The properties of the resulting formed article are shown in Table 2.

Comparative Example 12

An unstretched sheet was prepared in the same manner as in Example 8,and a container (formed article) was formed from the sheet insubstantially the same manner as in Example 8 by means of a formingmachine as employed in Example 8, except that the heat treatmentconditions were changed to perform the heat treatment in a die at 100°C. for 1 minute.

The properties of the resulting formed article are shown in Table 2.

The containers (formed articles) produced in Examples 8 to 15 were freefrom deformation even when hot water was poured therein, and wereexcellent in heat resistance. Further, the containers were excellent inshock resistance.

In Example 16, the sheet was prepared by employing the resin compositionwhich contained the polylactic acid having an optical purity within therange specified by the present invention and contained thearomatic/aliphatic copolymer polyester having a glass transitiontemperature of not higher than 0° C. and the talc in a blend ratiowithin the range specified by the present invention. After the sheet wassubjected to the heat treatment at a temperature for a period within theranges specified by the present invention, the article was formed fromthe sheet. Therefore, the resulting formed article had a highcrystallinity, and was excellent in heat resistance.

In Comparative Example 6, the talc was not employed, so that thecrystallization of the heat-treated container was insufficient.Therefore, the container was deformed in an instance when hot water waspoured in the container.

In Comparative Example 7, an excessively great amount of the talc wasadded, so that the container per se was embrittled. Therefore, crackswere observed in the container during the forming or after the forming.

In Comparative Example 8, the polylactic acid had a lower opticalpurity. Even through the crystallization was promoted by the heattreatment and the addition of the crystal nucleus agent, thecrystallization of the polylactic acid was insufficient and thecontainer was poorer in heat resistance.

In Comparative Example 9, the polyester (B) having a glass transitiontemperature of not higher than 0° C. was not employed as in Examples 1to 7. Therefore, the falling ball height was smaller, so that the shockresistance was insufficient as compared with Examples 8 to 16.

In Comparative Example 10, the polyester (B) having a glass transitiontemperature of not higher than 0° C. was blended in an excessively greatamount. Though the shock resistance was excellent, the crystallizationspeed was extremely low, resulting in a prolonged forming cycle. This isdisadvantageous from the viewpoint of industrial production.

In Comparative Example 11, the heat treatment temperature in the die was160° C., which was high and close to the melting point of the polylacticacid, so that crystal nuclei were fused. Therefore, the resultingcontainer was not sufficiently crystallized and, hence, was poorer inheat resistance.

In Comparative Example 12, the heat treatment temperature in the die was100° C., which was not high enough for the crystallization of moleculesof the polylactic acid. Even though the treatment period was prolonged,the crystallization was insufficient. Therefore, the container waspoorer in heat resistance with |ΔHm|−|ΔHc|=9.0 J/g.

1. A polylactic acid article formed from a sheet of a resin compositioncomprising polylactic acid as a major resin component and a crystalnucleus agent, the polylactic acid having an optical purity of not lowerthan 90% and a residual lactide amount of 0.1 to 0.6% by mass, thecrystal nucleus agent being present in a proportion of 1 to 25% by massin the resin composition, the formed article being characterized in thata difference (|ΔHm|−|ΔHc|) between an absolute value of a crystal fusionheat amount ΔHm as measured at a heat-up rate of 20° C./min by means ofa differential scanning colorimeter and an absolute value of a heat-upcrystallization heat amount ΔHc generated by heat-up crystallization isnot lower than 25 J/g, the formed article having a crystallinity of notlower than 35% as determined by X-ray measurement and a crystallizationspeed of not lower than 0.05 min⁻¹ at 130° C.
 2. A polylactic acidformed article as set forth in claim 1, wherein the crystal nucleusagent is talc having an average particle diameter of 0.1 to 10 μm.
 3. Apolylactic acid formed article as set forth in claim 1 or 2, wherein theresin composition further comprises a dispersant for the crystal nucleusagent, and the dispersant comprises an aliphatic amide.
 4. A polylacticacid formed article as set forth in claim 3, wherein the aliphatic amidecomprises at least one of erucamide, stearamide, oleamide,ethylene-bis-stearamide, ethylene-bis-oleamide andethylene-bis-laurylamide.
 5. A polylactic acid formed article as setforth in claim 1, which is formed by one of vacuum forming, air pressureforming, vacuum air pressure forming and press forming.
 6. A productionprocess for a polylactic acid article formed from a sheet of a resincomposition comprising polylactic acid as a major resin component and acrystal nucleus agent, the production process comprising the steps of:extruding the resin composition into a sheet, the polylactic acid in theresin composition having an optical purity of not lower than 90% and aresidual lactide amount of 0.1 to 0.6% by mass, the crystal nucleusagent being present in a proportion of 1 to 25% by mass in the resincomposition; heat-treating the sheet at a temperature of 110 to 150° C.for 1 to 30 seconds; and forming the sheet into the article after theheat treatment.
 7. A production process for a polylactic acid articleformed from a sheet of a resin composition comprising polylactic acid asa major resin component and a crystal nucleus agent, the productionprocess comprising the steps of: extruding the resin composition into asheet, the polylactic acid in the resin composition having an opticalpurity of not lower than 90% and a residual lactide amount of 0.1 to0.6% by mass, the crystal nucleus agent being present in a proportion of1 to 25% by mass in the resin composition; and forming the sheet intothe article while heat-treating the sheet at a temperature of 110 to150° C. for 1 to 30 seconds.
 8. A polylactic acid formed articleproduction process as set forth in claim 6 or 7, wherein one of vacuumforming, air pressure forming, vacuum air pressure forming and pressforming is employed for the forming.
 9. A polylactic acid article formedfrom a sheet of a resin composition comprising polylactic acid as amajor resin component, the resin composition comprising a crystallinepolylactic acid resin (A) having an optical purity of not lower than95%, an aromatic/aliphatic copolymer polyester or an aliphatic polyester(B) having a glass transition temperature of not higher than 0° C., andtalc (C) having an average particle diameter of 1 to 8 μm with an(A)/(B) blend ratio of (A)/(B)=97/3 to 80/20% by mass and with a (C)blend ratio of 1 to 30% by mass based on the total amount of thecomposition, the formed article having a crystallization index such thata difference between an absolute value of a crystal fusion heat amountΔHm as measured at a heat-up rate of 20° C./min by means of adifferential scanning calorimeter and an absolute value of a heat-upcrystallization heat amount ΔHc is (|ΔHm|−|ΔHc|)≧25 J/g, acrystallization speed of not lower than 0.010 min⁻¹ at 130° C., and afalling ball impact resistance such that a falling ball height is notsmaller than 20 cm with respect to a thickness of 500 μm.
 10. Apolylactic acid formed article as set forth in claim 9, which is formedby one of vacuum forming, air pressure forming, vacuum air pressureforming and press forming of the sheet.
 11. A production process for apolylactic acid article formed from a sheet of a resin compositioncomprising polylactic acid as a major resin component, the productionprocess comprising the steps of: extruding the resin composition into asheet, the resin composition comprising a crystalline polylactic acidresin (A) having an optical purity of not lower than 95%, anaromatic/aliphatic copolymer polyester or an aliphatic polyester (B)having a glass transition temperature of not higher than 0° C., and talc(C) having an average particle diameter of 1 to 8 μm with an (A)/(B)blend ratio of (A)/(B)=97/3 to 80/20% by mass and with a (C) blend ratioof 1 to 30% by mass based on the total amount of the composition;heat-treating the sheet at a treatment temperature of 110 to 150° C. fora treatment period of 1 to 30 seconds and forming the sheet into thearticle.
 12. A polylactic acid formed article production process as setforth in claim 11, wherein the sheet is formed by one of vacuum forming,air pressure forming, vacuum air pressure forming and press formingafter the sheet is heat-treated.
 13. A polylactic acid formed articleproduction process as set forth in claim 11, wherein the sheet is formedby one of vacuum forming, air pressure forming, vacuum air pressureforming and press forming, while the sheet is heat-treated in a die.